1. Field of the Invention
This invention relates to printing or rapidly transferring fixed servo reference data to surfaces such as those of magnetic storage media and, more particularly, to printing servo data with compound phase patterns that provide absolute position information, that are well suited to commercial data channel chips, and that require relatively little space on the recording surface.
2. Description of the Prior Art
Modern magnetic recording systems have servo information or position markers written in an interleaved fashion on the same surface on which data are recorded. To simplify the system the same head is used to read the user data and the servo information. Various special formats are used for the servo information to enable using measurements and subsequent signal processing to determine the position of the read head relative to the center of the desired data track. A head movement mechanism and servo control system keep the read head close enough to the data track center to assure reliable reading and writing of user data.
Early hard disk systems used a special machine, a servo track writer (STW), e.g. U.S. Pat. No. 5,333,140, to record the servo information on the disk surface. The STW includes a clamping system to hold the hard disk drive (HDD) in a fixed reference position and an external motor with a laser or optical encoder to accurately move a reference pin that extended into the HDD. The actuator or head positioning mechanism of the HDD is biased against the pin so the write head of the HDD can be placed at any desired radius by moving the external motor according to its encoder system. The STW also includes a clock head that is temporarily placed on a surface of the disk by means of a special aperture in the HDD case. Circuitry of the STW writes a clock signal or timing reference by applying a pattern of write current to the clock head. The clock head reads the timing reference signal as the HDD write head is moved to any desired radius. Since the timing reference is fixed relative to the disk it is possible to write servo patterns of a desired form as function of radius and angle on the disk. Related STW are used for removable media such as the ZIP floppy diskette system manufactured by Iomega Corporation.
The STW must turn the disk through at least one revolution to write the servo information, and another fraction of a revolution is required to move the head to the next radius. Many servo patterns use the edges of special bursts or sub elements in the determination of the position from the read back signal, so it is necessary to write the servo bursts at radial displacements of a fraction of the data track width. Therefore it is usually required to write two or more servo tracks for each data track. Since HDDs now have about one hundred thousand or more data tracks, it may require tens of minutes to write the servo pattern.
Because the enclosure of the HDD is required to have openings for the clock head and for the reference pin it is necessary to use the STW in a special clean room to avoid contamination of the head-media interface. It is expensive and difficult to maintain the complex STW in such a clean environment.
A new approach was offered by a “printing” method (Ishida, T., et al., “Printed Media Technology for an Effective and Inexpensive Servo Track Writing of HDDs”, IEEE Trans. Magn., p 1875, 2001 and Sugita, R., et al., “A Novel Magnetic Contact Duplication Technique for Servo-Writing on Magnetic Disks”, IEEE Trans. Magn. p 2285, 2001). In that method the desired servo pattern is replicated in a “master disk” consisting of a silicon substrate about one millimeter thick with strips of highly permeable cobalt about one half micron thick embedded in the silicon. The face of the master containing the cobalt elements is placed in contact with a D.C. erased slave disk. Then a permanent magnet producing an oppositely directed field is brought close to the back surface of the master and is rotated one revolution relative to the master-slave pair. The cobalt elements shield portions of the slave disk leaving them in the original D.C. state, but gaps in the cobalt pattern allow the field to penetrate. The field is concentrated at the gaps and the increased fringing components reverse the magnetization of the adjacent portions of the slave disk. This rapid transfer of the pattern to the entire surface of the slave disk, or “printing”, is done as the last step at the end of a conventional disk manufacturing line.
Important feature sizes, typically line widths and the thickness of cobalt elements, have been steadily decreasing, but the transition density of printing currently lags that of conventional write heads. Direct printing of a conventional edge of burst servo pattern resulted in poor performance at contemporary data densities, (Ishida, et al., “Demodulation of servo tracking signals printed with a lithographically patterned master disk”, IEEE Trans. Magn., Vol. 37, No. 4, July, 2001, pp 1412–1415).
The conventional Gray codes would provide the absolute radial position, but they are not well suited to the printing process and result in fuzzy transitions at track edges. Unfortunately those codes occupy a large part of the surface, they are not well adapted to the architecture of commercial channel chips, and they require larger variations of widths of lines and spaces. The large variations of feature sizes exacerbate noise sources of the printing process. Proper choice of geometry including width and thickness of the cobalt elements and width of the gaps is necessary to assure magnetic switching of the slave medium next to the gaps of the master without saturating the cobalt film to produce “secondary gaps” and consequent writing of spurious pulses or noise (Saito, A., et al., “Magnetic printing technique for longitudinal thin film media with high coercivity of 6000 Oe”, J. Appl. Phys., V 91, p 8688, 2002 and Baker, “Tradeoffs for magnetic printing of servo patterns”, J. Appl. Phys., p 8691, 2002).
Therefore it was proposed in U.S. Pat. No. 6,304,407 to use the printed pattern as a reference system for self-servowriting (SSW). Because the printing method involves several processes such as optical diffraction, diffusion in the photo resist, and shadowing during sputtering of the cobalt, it is difficult to produce square corners or small radii of curvature at the ends of the cobalt lines. Therefore phase methods are used as in U.S. Pat. No. 3,686,649 for the position information, and the phase is measured by discrete Fourier transforms (DFT) in the manner of U.S. Pat. No. 5,784,296. In this method the ends of the bars in the inclined phase elements are excluded from the sample window, and pulses are measured at the long, clean edges of magnetic transitions.
After assembly of the HDD it is removed from the clean room and placed on a self-test rack where it begins its self-servowriting directed by the embedded firmware. Well known self-test methods measure possible pattern eccentricity and any minor errors of the position information for each servo block printed on the disk. Then corrections are applied for subsequent writing of a final servo pattern. The relatively low additional cost of printing one surface of a disk eliminates the need for an expensive STW and the clean room in which to operate it.
Mitsubishi Corporation using well-known magneto-optic (MO) techniques subsequently developed an alternate printing method. The same pattern described above is first replicated as a chromium on glass reticle or mask. Such masks are commonly created by photolithography. Opaque features are left as a thin chromium film on a transparent glass substrate. In this case too the slave disk is initially D.C. erased, and the printing magnet applies a field in the opposite direction. The applied field is a bit lower than the coercivity of the slave so its magnetization is preserved until a laser is flashed through the mask. The brief laser pulse heats areas under gaps of the pattern. The surface temperature of the recording film rises quickly decreasing the local coercivity and allowing the selected regions to switch magnetization directions.
This contact magneto-optic method also has limitations due to diffraction and to the difficulty of maintaining the small space between reticle and slave disk and due to reflections between the two. Using an antireflective coating on the reticle is difficult because the energy density of the laser irradiation damages the coating. Some of these problems are ameliorated by a projection printing demonstration (Wang, L., et al., “Photo thermal patterning on magnetic media”, J. Appl. Phys., V 91, p 8685, 2002).
As is well known in the disk drive industry the number of servo wedges or position bearing segments of the disk must increase as the track density increase. Drives now have a few hundred wedges and the trend is toward higher densities. The patterns are made by various processes such as fine scale lithography, which is also used in the manufacture of semiconductors and read-write heads for disk drives.
The SSW process described above utilized early printed disks when the critical feature sizes or printable line and space widths were greater than one micron. There was no absolute position information, but it was adequate to slowly move the head in small steps along the radial extent of the HDD to write each of the final servo tracks. The relatively large printed reference patterns are simply overwritten after the final pattern has been completed. It is also well known that fields at the edges of conventional write heads are poorly controlled, (Van Herk, “Analytical expressions for side fringing response and crosstalk with finite head and track widths”, IEEE Trans. Magn., Vol. 13, No. 6, November, 1977, pp 1764–1766 and Tsang, et al., “Disk-noise induced peak jitters in high density recording”, IEEE Trans. Magn., Vol. 29, No. 6, November, 1993, pp 3975–3977.) Therefore the STW and the SSW method both introduce noise at edges of bursts of conventional final servo patterns. That noise becomes a greater problem at extremely high track densities.